The 1000 MW installed pump/generating capacity of the proposed Lake Onslow Scheme is not unusual by world standards. However, its energy storage capacity (up to 5 TWh) would appear to make it the world's largest pumped storage scheme by energy storage measure. The unusual ratio between energy storage and installed capacity comes about from New Zealand's special hydro generation situation. Hydro is the dominant mode of power generation but national hydro storage capacity is relatively limited. This makes hydro electricity output vulnerable to extended dry periods. Developed to 5 TWh of storage, the large Lake Onslow Scheme would more that double New Zealand's total hydro storage capacity. The scheme would thus provide dry period insurance as well carrying out the usual short-term pumping and generating operations. It would be an open-cycle system. The lower reservoir is the Clutha River, which is New Zealand's largest river by discharge measure.

It seems a long time ago now since I first proposed the possibility of pumped storage at Lake Onslow. But just confirming - having the equivalent amount of energy (4-5 TWh) in batteries would be impossibly expensive. Also, the batteries would need to be replaced from time to time.